Physical layer security technology has become an effective solution to the wireless communication security problem. As shown in Figure .REF , the transmitter (i.e., Alice) sends a confidential message to the legitimate receiver (i.e., Bob), while the eavesdropper (i.e., Eve) receives the signal and intends
to decode it. The key idea of physical layer security technology is to exploit the inherent propagation characteristics of wireless channels (such as the difference between the main channel and the eavesdropping channel, randomness, and reciprocity) from the perspective of information theory.
By reasonably designing the transmit signal so that it improves the amount of mutual information between the transmitter and the desired user at the physical layer while reducing the amount of information in the eavesdropping channel. Therefore, the physical layer security technology can effectively protect the content of private messages sent by the transmitter from being eavesdropped by illegal eavesdroppers [1]}. This improves the privacy and security of the transmitter's communication with other legitimate users. Compared with traditional encryption technology, physical layer security technology has the following notable features and advantages: First, the physical layer security technology can achieve keyless security, that is, no encryption and decryption operations are required. Second, physical layer security techniques can take advantage of the time-varying and random nature of wireless channels. Signal processing techniques are employed to design reasonable beamforming or power allocation strategies from the transmitter's perspective to improve the security performance of wireless communication systems [2]}. Based on the above advantages, solving the communication security problem from the physical layer has aroused widespread concern.
The authors in [3]} studied the secure transmission of private information on Gaussian channels, and proved that expanding the difference between the main channel and the eavesdropping channel can achieve low probability interception and low probability detection for eavesdroppers.
[4]} further researched the keyless physical layer secure transmission technology over fading channels.
The use of multiple antennas can add additional degrees of freedom and further improve the security performance of the wireless network [5]}, [6]}. In addition, by generating random artificial noise (AN) at the transmitter to interfere with eavesdroppers, the security of the system can be further improved [7]}.
Currently, scenarios where eavesdroppers exist are considered in various wireless communication systems.
